1. Field of the Invention
The present invention-relates to an image display capable of performing multi-gradation display and, more particularly, to an image display suitable for high gradation display.
2. Description of the Related Art
Referring to FIGS. 16 to 18, two conventional techniques will be described hereinbelow.
FIG. 16 is a configuration diagram of a light emitting device using a first conventional technique (hereinbelow, called first conventional technique). Pixels 205 each having an organic electro-luminescent (EL) element 204 as a pixel light emitting material are disposed in a matrix in a display part. The pixels 205 are connected to external drive circuits via gate lines 206, source lines 207, power source lines 208, and the like. In each pixel 205, the source line 207 is connected to the gate of a power TFT 203 and one end of a storage capacitor 202 via a logic TFT (Thin-Film-Transistor) 201, and one end of the power TFT 203 and the other end of the storage capacitor 202 are commonly connected to the power source line 208.
The other end of the power TFT 203 is connected to a common power source terminal via the organic EL element 204. One end of the gate line 206 is connected to a frame scanning circuit 210, and one end of the source line 207 is connected to an analog signal voltage input circuit 209. The logic TFT 201 and the power TFT 203 are formed by using polysilicon TFTs on an SiO2 substrate.
The operation of the first conventional technique with such a configuration will now be described.
When the logic TFT 201 in a predetermined pixel row is opened/closed by the frame scanning circuit 210 via the gate line 206, an analog signal voltage supplied from the analog signal voltage input circuit 209 to the source line 207 is supplied to the gate of the power TFT 203 and the storage capacitor 202 and held for a period of one frame until the next scan writing is performed. The power TFT 203 supplies an analog signal current according to the analog signal voltage to the organic EL element 204. It makes the organic EL element 204 emit light with brightness corresponding to the analog signal voltage.
The first conventional technique is disclosed in detail in, for example, Japanese Unexamined Patent Application No. 8-241048. Although the term “organic electro-luminescent (EL) element” is used as the light emitting element in the above description of the conventional technique in accordance with the term used in the publication in the conventional technique, in recent years, the light emitting element is often called an OLED (Organic Light Emitting Diode) device. In the specification as well, the latter term will be used hereinbelow.
Referring now to FIGS. 17 and 18, another conventional technique will be described.
FIG. 17 is a configuration diagram of a light emitting device using a second conventional technique (hereinbelow, called a second conventional technique). The structure of the second conventional technique is basically similar to the structure described in the first conventional technique except that a digital signal voltage input circuit 211 is provided in place of the analog signal voltage input circuit 209 and a sub frame scanning circuit 212 is provided in place of the frame scanning circuit 210. Only the difference in operations due to the structural differences will be described.
Referring to FIG. 18, the operation of the second conventional technique will be described. As shown in FIG. 18, in the conventional technique, one frame period for displaying information of one frame is divided into a plurality of sub frame periods. Further, the sub frame period is constructed by an address period Ts as a period of writing a display signal to each pixel and each of sustain periods T1 to Tn (to simplify explanation, n=5 in FIG. 18) for performing display with or without light emission in accordance with a written display signal. In the address periods Ts, the drive voltage of the OLED device is at the off level and light is not emitted. Although the operation of writing a display signal to each pixel in each address period is basically similar to that of the first conventional technique, the display signal is not an analog signal voltage but a digital signal voltage of two values of “high level” and “low level”.
Therefore, light emission of the OLED device in each of the sustain periods T1 to T5 subsequent to the address periods Ts is also digital light emission of “on” or “off”. As shown in FIG. 18, a time weight of the i-th power of 2 is assigned to each of the sustain periods T1 to T5 of the sub frames, so that a weight is assigned to each light emission bit. It enables gray scale display according to each of bits of digital data in the second conventional technique.
An advantage of the conventional technique is that since the power TFT 203 is used as a simple switch, a characteristic variation of the power TFT 203 such as a threshold voltage is not reflected in brightness at the time of light emission. In the conventional technique, consequently, an image can be displayed with small brightness variation and high picture quality. Such a conventional technique is disclosed in, for example, Japanese Unexamined Patent Application No. 2001-159878.